I just finished Bill Bryson's A Short History of Nearly Everything yesterday, and I have to say that it was an excellent book. It is a pretty substantial text, but Bryson does a good job of keeping it engaging, informative, and understandable even without background knowledge in the areas he addresses. I highly recommend it (as does Sarah, who recommended it to me in the first place).
The book, as the title suggests, covers quite a sweep of subjects. What Bryson is essentially attempting to do is explain the history of the Earth and life on it, but to do so he attempts to relate how and why we believe it. In doing so, he humanizes science through a series of fascinating historical anecdotes about scientists both famous and obscure. The devotion that Bryson lends to tracing the development of ideas is, I think, the greatest strength of the book. By examining how we know what we know, he successfully elucidates the nature of the scientific manner in an engaging and colourful manner. I wish this sort of book were presented more often in a middle school science class, as I think it helps bring to life the scientific mindset much more effectively than memorizing the (usually misrepresented) structure of hypothesis, data collection, conclusion.
Of course, with such a sweeping book there are bound to be a few errors. The only one I can remember was he accidentally listed Parkinson's Disease as being caused by a single genetic defect, which is not actually true (the origins of Parkinson's Disease are not currently known). That is a very minor quibble, though, and does not at all detract from the overall message of the book.